Method for manufacturing semiconductor light emitting device

ABSTRACT

A method for manufacturing a semiconductor light emitting device is provided. The device includes: an n-type semiconductor layer; a p-type semiconductor layer; and a light emitting unit provided between the n-type semiconductor layer and the p-type semiconductor layer. The method includes: forming a buffer layer made of a crystalline Al x Ga 1-x N (0.8≦x≦1) on a first substrate made of c-plane sapphire and forming a GaN layer on the buffer layer; stacking the n-type semiconductor layer, the light emitting unit, and the p-type semiconductor layer on the GaN layer; and separating the first substrate by irradiating the GaN layer with a laser having a wavelength shorter than a bandgap wavelength of GaN from the first substrate side through the first substrate and the buffer layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefits of priority fromthe prior Japanese Patent Application No. 2008-218028, filed on Aug. 27,2008; the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing asemiconductor light emitting device.

2. Background Art

A GaN-based mixed crystal such as AlGaInN and the like has a largebandgap of a direct transition type, and is used as a material for lightemitting devices (for example, LED: light emitting diode) of shortwavelength.

Currently, since there is no good substrate which is lattice-matchedwith a GaN-based mixed crystal, a method is used for convenience thatgrows a GaN-based mixed crystal on a sapphire substrate via alow-temperature-grown amorphous layer or a polycrystal-form bufferlayer.

At this time, nearly half of the emitted light is reflected at theinterface between the sapphire and the GaN-based mixed crystal becausethere is a difference in refractive index between the sapphire and theGaN-based mixed crystal, causing a decrease in efficiency. Accordingly,a method in which the substrate is separated by an irradiation of ahigh-power laser with a short wavelength from the sapphire side todecompose the GaN facing the substrate is tried.

However, this method causes damage of the device structure unit due toheat and/or stress during the laser irradiation, and therefore decreasesefficiency and/or generates high-density threading dislocations due tolattice mismatching, leading to degradation of device characteristics.Further, a crack easily occurs because of residual strain, causing adecrease in yield.

These problems are significant particularly for an ultraviolet region ofwavelengths shorter than 400 nm (nanometers) in which crystal defectsoften influence efficiency, even in a region of wavelengths longer than370 nm in which the light is not so absorbed by GaN.

Japan Patent No. 3803606 discloses a method that forms athermal-diffusion control layer having a lower thermal conductivity thana group-III nitride semiconductor which forms a first semiconductorlayer, and irradiates light beam which is absorbed in the firstsemiconductor layer to decompose the first semiconductor layer.

SUMMARY OF THE INVENTION

According to an aspect of the invention, there is provided a method formanufacturing a semiconductor light emitting device, the deviceincluding: an n-type semiconductor layer; a p-type semiconductor layer;and a light emitting unit provided between the n-type semiconductorlayer and the p-type semiconductor layer, the method including: forminga buffer layer made of a crystalline Al_(x)Ga_(1-x)N (0.8≦x≦1) on afirst substrate made of c-plane sapphire and forming a GaN layer on thebuffer layer; stacking the n-type semiconductor layer, the lightemitting unit, and the p-type semiconductor layer on the GaN layer; andseparating the first substrate by irradiating the GaN layer with a laserhaving a wavelength shorter than a bandgap wavelength of GaN from thefirst substrate side through the first substrate and the buffer layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating a method formanufacturing a semiconductor light emitting device according to anembodiment of the present invention;

FIG. 2 is a flowchart illustrating the method for manufacturing asemiconductor light emitting device according to the embodiment of thepresent invention;

FIG. 3 is a schematic cross-sectional view in order of the steps,illustrating a method for manufacturing a semiconductor light emittingdevice according to a first example of the present invention;

FIG. 4 is a schematic cross-sectional view in order of the stepsfollowing FIG. 3;

FIG. 5 is a schematic cross-sectional view in order of the stepsfollowing FIG. 4; and

FIG. 6 is a schematic cross-sectional view in order of the steps,illustrating a method for manufacturing a semiconductor light emittingdevice according to a second example of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will now be described withreference to the drawings.

The drawings are schematic or conceptual; and the relationships betweenthe thickness and width of portions, the proportions of sizes amongportions, and the like are not necessarily the same as the actual valuesthereof. Further, the dimensions and proportions may be illustrateddifferently among drawings, even for identical portions.

In the specification and drawings, components similar to those describedin regard to a drawing thereinabove are marked with like referencesigns, and a detailed description is omitted as appropriate.

Embodiments

FIG. 1 is a schematic cross-sectional view illustrating a method formanufacturing a semiconductor light emitting device according to anembodiment of the present invention.

FIG. 2 is a flowchart illustrating the method for manufacturing asemiconductor light emitting device according to the embodiment of thepresent invention.

As shown in FIG. 1, a semiconductor light emitting device 10manufactured by the method for manufacturing a semiconductor lightemitting device according to the embodiment of the present inventionincludes an n-type semiconductor layer 130, a p-type semiconductor layer150, and a light emitting unit 140 provided between the n-typesemiconductor layer 130 and the p-type semiconductor layer 150.

As shown in FIG. 1 and FIG. 2, in the method for manufacturing thesemiconductor light emitting device according to the embodiment, first abuffer layer 120 made of crystalline Al_(x)Ga_(1-x)N (0.8≦x≦1) and a GaNlayer 123 are stacked on a first substrate 110 made of c-plane sapphire(step S110). That is, the buffer layer 120 is formed on the firstsubstrate 110 and the GaN layer 123 is formed on the buffer layer 120.

Then, the n-type semiconductor layer 130, the light emitting unit 140,and the p-type semiconductor layer 150 are stacked on the GaN layer 123(step S120).

A p-side electrode 160 (not shown) may be formed on a side of the p-typesemiconductor layer 150 opposite to the light emitting unit 140, forexample.

A second substrate 180 may be bonded on the side of the p-typesemiconductor layer 150 opposite to the light emitting unit 140, namely,for example, on the p-side electrode 160.

Subsequently, the GaN layer 123 is irradiated with a laser 190 having awavelength shorter than a bandgap wavelength of GaN from the firstsubstrate 110 side through the first substrate 110 and the buffer layer120 to separate the first substrate 110 (step S130). The bandgapwavelength of GaN is defined by a bandgap energy of GaN.

A harmonic wave of a vanadium-oxide-type solid laser with a wavelengthof 355 nm, for example, may be used for the laser 190. The laser 190passes through the first substrate 110 made of sapphire and the bufferlayer 120, and is absorbed in the GaN layer 123 proximal to theinterface with the buffer layer 120. The generated heat partiallydecomposes the GaN layer 123. Consequently, the first substrate 110 isseparated from the light emitting unit 140 side.

Thereby, crystal damage during the separation of the first substrate 110made of sapphire is suppressed, and therefore a method for manufacturinga semiconductor light emitting device of short wavelength with a highyield, low cost, and high efficiency is provided.

In conventional art, a GaN layer is grown via a low-temperature-grownamorphous layer or a polycrystal-form buffer layer. More specifically,the low-temperature-grown amorphous layer or the polycrystal-form bufferlayer is inserted between the GaN layer and the substrate. When the GaNlayer is irradiated with a laser from the substrate side in such aconfiguration, the generated heat is transferred to the light emittingunit 140 side because the thermal conductivities of the sapphire and thebuffer layer are much lower than that of the GaN layer. Accordingly,efficiency easily decreases due to the crystal damage.

In contrast, in the method for manufacturing the semiconductor lightemitting device according to the embodiment, the crystalline bufferlayer 120 made of Al_(x)Ga_(1-x)N with high thermal conductivity isprovided between the first substrate 110 and the GaN layer 123.Accordingly, heat 195 generated during the irradiation of the GaN layerwith the laser 190 from the first substrate 110 side rapidly diffuses tothe buffer layer 120 side. Thereby, the decrease in efficiency due tocrystal damage can be prevented even when a high-power laser is used.Therefore, the decrease in efficiency due to crystal damage can besuppressed.

In other words, a process time can be reduced because problems such as adecrease in emitting efficiency can be avoided even when a high-powerlaser is used.

Further, since a laser with a wavelength longer than 350 nm which is notso absorbed in the GaN layer 123 can be used so that a process-denaturedportion is not localized in a thin region, processing strain is relaxed,the defects such as cracks are reduced, and the yield is increased.

As described above, the embodiment provides a method for manufacturing asemiconductor light emitting device of short wavelength with highproductivity and high efficiency while suppressing crystal damage duringthe separation of the sapphire substrate.

The inventors have found out that, by forming a high-temperature-grownlayer containing high-concentration carbon or hydrogen on the firstsubstrate 110 in place of the conventional low-temperature-grown AlN,GaN, or the like, a thick AlGaN or AlN film with a high-Al compositioncan be formed thereon, the crystal quality of the GaN layer growingthereon can be significantly improved, and a light emitting device withhigh efficiency can be fabricated. When the GaN layer 123 of a devicewafer fabricated by using this method is irradiated with the laser 190from the first substrate 110 side, thermal damage to the devicestructure unit is suppressed because the generated heat is rapidlyabsorbed in the AlN or AlGaN having high thermal conductivity, and thegeneration of cracks due to the thermal stress is also suppressedbecause there are few defects in the GaN layer.

More specifically, during the formation of the buffer layer in themethod for manufacturing the semiconductor light emitting deviceaccording to the embodiment, a first buffer layer containing carbon anda second buffer layer having a lower carbon concentration than the firstbuffer layer are stacked on the first substrate 110.

Thereby, the damage during the separation of the substrate by the laserirradiation can be reduced, and therefore a light emitting device of, inparticular, an ultraviolet region with wavelengths shorter than 400 nmcan be manufactured with a high yield and low cost, which has been seenas difficult to manufacture. Thereby, a peak wavelength of emitted lightof the light emitting unit 140 of the semiconductor light emittingdevice 10 manufactured by the method according to the embodiment of thepresent invention can be longer than 370 nm and shorter than 400 nm.

The crystalline buffer layer 120 made of Al_(x)Ga_(1-x)N with highthermal conductivity is formed at high temperatures of, for example,1100° C. or higher.

In the specification of the present application, a “crystalline” statemeans a state of neither amorphous nor polycrystalline.

FIRST EXAMPLE

A first example according to the embodiment will now be described.

FIG. 3 is a schematic cross-sectional view in order of the steps,illustrating a method for manufacturing a semiconductor light emittingdevice according to a first example of the present invention.

FIG. 4 is a schematic cross-sectional view in order of the stepsfollowing FIG. 3.

FIG. 5 is a schematic cross-sectional view in order of the stepsfollowing FIG. 4.

As shown in FIG. 3, firstly, a first buffer layer 121, a second bufferlayer 122, a GaN layer (a lattice relaxation layer) 123, an n-typecontact layer 131, an n-type cladding layer 132, a light emitting unit140, a spacer layer 143, a p-type cladding layer 151, and a p-typecontact layer 152 are sequentially stacked on a first substrate 110 ofwhich a surface is a sapphire c-plane, by using metalorganic chemicalvapor deposition.

In the above, the first buffer layer 121 is a high-carbon-concentrationportion. It may be based on AlN, for example, and have a carbonconcentration of 3×10¹⁸ cm⁻³ to 5×10²⁰ cm⁻³ and a layer thickness of 3nm to 20 nm, for example.

The second buffer layer 122 may be based on high-purity AlN, forexample, and have a carbon concentration of 1×10¹⁶ cm⁻³ to 3×10¹⁸ cm⁻³and a layer thickness of 0.6 μm (micrometers) to 6 μm, for example.

The GaN layer (the lattice relaxation layer) 123 may be based onnon-doped GaN, and have a layer thickness of 2 μm, for example.

The n-type contact layer 131 may be based on Si-doped n-type GaN, andhave an Si concentration of 1×10¹⁹ cm⁻³ to 2×10¹⁹ cm⁻³ and a layerthickness of 4 μm, for example.

The n-type cladding layer 132 may be based on Si-doped n-typeAl_(0.13)Ga_(0.8)N, for example, and have an Si concentration of 2×10¹⁸cm⁻³ and a layer thickness of 0.02 μm, for example.

The light emitting unit 140 may include: a multiple-quantum-wellstructure with barrier layers 141 and well layers 142 which arealternately stacked at six times; and a terminal barrier layer 141 a.

The barrier layer 141 may be based on Si-doped n-typeAl_(0.065)Ga_(0.93)In_(0.005)N, for example, and have an Siconcentration of 1.1×10¹⁹ to 3.0×10¹⁹ cm⁻³ and a layer thickness of 13.5nm, for example.

The well layer 142 may be based on GaInN, for example, and have anemission wavelength of 375-385 nm and a layer thickness of 4.5 nm, forexample.

The terminal barrier layer 141 a may be based on Si-doped n-typeAl_(0.065)Ga_(0.93)In_(0.005)N, for example, and have an Siconcentration of 1.1×10¹⁹ to 3.0×10¹⁹ cm⁻³ and a layer thickness of 4.5nm, for example.

The spacer layer 143 may be based on low-Si-concentrationAl_(0.065)Ga_(0.93)In_(0.005)N, for example, and have an Siconcentration of 1×10¹⁶ cm⁻³ to 3.0×10¹⁸ cm⁻³ and a layer thickness of4.5 nm, for example.

The p-type cladding layer 151 may be based on Mg-doped p-typeAl_(0.24)Ga_(0.76)N, for example, and have an Mg concentration of1.8×10¹⁹ cm⁻³ on the spacer layer 143 side and 1×10¹⁹ cm⁻³ on the sideopposite to the spacer layer 143, and a layer thickness of 24 nm, forexample.

The p-type contact layer 152 may be based on Mg-doped p-type GaN, forexample, and have an Mg concentration of 8×10¹⁸ cm⁻³ on the p-typecladding layer 151 side and 5×10¹⁹ to 9×10¹⁹ cm⁻³ on the side oppositeto the p-type cladding layer 151, and a layer thickness of 0.05-0.3 μm,for example.

The buffer layer 120 illustrated in FIG. 1 includes the first bufferlayer 121 and the second buffer layer 122.

The n-type semiconductor layer 130 illustrated in FIG. 1 includes then-type contact layer 131 and the n-type cladding layer 132.

The p-type semiconductor layer 150 illustrated in FIG. 1 includes thep-type cladding layer 151 and the p-type contact layer 152.

Subsequently, depressions 153 are formed on the surface of the p-typecontact layer 152 at spacings of about 0.1 μm which is not longer thanthe wavelength of the light emitted from the light emitting unit 140.The depressions 153 have a width of about 0.02 μm and a depth of about0.1 μm, for example. Thus, projections 154 or depressions 153 are formedon the surface of the p-type contact layer 152 at a spacing which is notlonger than the wavelength of the light emitted from the light emittingunit 140. The spacing is an average spacing of the projections 154 or anaverage spacing of the depressions 153.

For example, the depressions 153 may be formed by vapor-phase etching byusing a polymer film with a self-aligning function as a patterning mask,in addition to by lithography. Forming the depressions 153 can improvethe efficiency of light extraction, and enhance the adhesion of a silverelectrode which is hard to form an alloy and easily removed.

Then, an SiO₂ film is stacked with a thickness of 400 nm over thesemiconductor layer by using, for example, a thermal CVD apparatus.

Subsequently, to form a p-side electrode 160, first a patterned resistfor a resist-lift-off process is formed on the semiconductor layer, andthe SiO₂ film on the p-type contact layer 152 is removed by a treatmentwith ammonium hydrogen fluoride. In the region where the SiO₂ film isremoved, an Ag film of a reflecting electrode serving as the p-sideelectrode 160 is formed with a film thickness of 200 nm by using avacuum evaporation apparatus. Then, a sinter treatment is performed forone minute under a nitrogen gas atmosphere at 350° C. Thus, the p-sideelectrode 160 mainly made of silver and a dielectric film 155 (an SiO₂film) provided therearound which has a function of a passivation film aswell are formed on the wafer.

After that, a pad layer 161 mainly made of gold is formed on the surfaceof the p-side electrode 160 and the dielectric film 155 (the SiO₂ film)to cover the p-side electrode 160 and the dielectric film 155 (the SiO₂film).

As illustrated in FIG. 4, an Si substrate serving as a second substrate180 on which a conductive layer 181 made of gold was previously providedby, for example, vapor deposition is disposed so that the conductivelayer 181 and the pad layer 161 may be opposed to each other. Then,these components are pressure-bonded to each other while heating them.Thereby, a stacked body including the light emitting unit 140 and thesecond substrate 180 composed of the Si substrate are bonded to eachother. That is, the second substrate 180 is bonded on the side of thep-type semiconductor layer 150 opposite to the light emitting unit 140.

At this time, a crack and the like usually easily occur due to adifference in thermal expansion between Si and GaN, and a difference inresidual stress between the protective film (the dielectric film 155)and the electrode materials (the p-side electrode 160, the metal(conductive layer) 181, and the pad layer 161). However, since thisexample uses a crystal with low threading dislocation concentration,such a problem does not occur.

Then, as shown in FIG. 5, pulsed light of a vanadium-oxide-based solidlaser (355 nm) as the laser 190 is irradiated from the first substrate110 side while scanning. The laser is transmitted through the firstsubstrate 110 made of sapphire and the buffer layer 120 (the firstbuffer layer 121 and the second buffer layer 122) made of AlN, andabsorbed in the GaN layer 123 proximal to the interface with the bufferlayer 120. The GaN layer 123 is decomposed partially by the generatedheat.

Subsequently, the decomposed portion is removed by a treatment with warmhydrochloric acid and the like, and the first substrate 110 made ofsapphire is separated.

In the case where a wafer with a GaN layer formed therein via anordinary low-temperature-grown buffer layer of thin film is irradiatedwith a laser, the generated heat is transferred to the light emittingunit 140 side because the thermal conductivity of sapphire is much lowerthan that of the GaN layer. Therefore, efficiency easily decreases dueto the crystal damage.

In contrast, in the method for manufacturing the semiconductor lightemitting device of the example, the AlN layer (the first buffer layer121 and the second buffer layer 122) with high thermal conductivity isformed on the first substrate 110 side. Accordingly, the generated heatrapidly diffuses to the AlN layer side. Therefore, problems such as adecrease in emitting efficiency do not occur even when a high-powerlaser is used, and a processing time can be reduced. Further, because alaser with a wavelength longer than 350 nm which is not so absorbed inthe GaN layer 123 can be used so that the process-denatured portion isnot localized in a thin region, processing strains are relaxed, anddefects such as cracks are reduced.

In the example, the first buffer layer 121 relaxes the difference incrystal type with the first substrate 110, and in particular reducesscrew dislocations.

The surface of the second buffer layer 122 is planarized at the atomiclevel. Therefore, defects in the GaN layer (the lattice relaxationlayer) 123 growing thereon are reduced. To reduce the defects, it ispreferable that the second buffer layer 122 has a film thickness of morethan 0.8 μm. In addition, to prevent a warpage due to strain, it ispreferable that the second buffer layer 122 has a film thickness of notmore than 4 μm. From the viewpoint of repeatability and productivity, itis more preferable that the second buffer layer 122 has a film thicknessof 1.5-3 μm.

The second buffer layer 122 is preferably based on AlN in regard tothermal conductivity. However, Al_(x)Ga_(1-x)N (0.8≦x≦1) may also beused, which can compensate a warpage of the wafer by adjusting the Gacontent and is favorable to a large-scale wafer.

The GaN layer (the lattice relaxation layer) 123 functions to reducedefects and relax strain by growing in the form of a three-dimensionalisland on the second buffer layer 122. To planarize the surface ofgrowth, the lattice relaxation layer 123 needs to have an average filmthickness of 0.6 μm or more. From the viewpoint of repeatability andproductivity, it is preferable that the lattice relaxation layer 123 hasa film thickness of 0.8 μm to 2 μm.

Employing the buffer layer 120 (the first buffer layer 121 and thesecond buffer layer 122) and the GaN layer (the lattice relaxationlayer) 123 can decrease the density of dislocation to 1/10 or lesscompared to conventional low-temperature-grown buffer layers.

This allows crystal growth at a high temperature, which are usuallydifficult to use because of anomalous growth, and at a high ratio of (asupply rate of group V atoms)/(a supply rate of group III atoms).Therefore, the generation of point defects is suppressed, andhigh-concentration doping to AlGaN and the barrier layer having ahigh-Al composition is possible.

In this example, to obtain high-efficiency light emitting in theultraviolet region while utilizing the advantage of a low-defectcrystal, various schemes that enable to employ the p-type cladding layer151 with a high-Al composition and a thick film thickness are adopted inorder to increase efficiency of the light emitting unit 140 itself andprevent electrons from overflowing from the light emitting unit 140.

By doping the barrier layer 141 with high-concentration Si andincreasing the electron concentration in the well layer 142, a radiativerecombination lifetime is decreased and the efficiency is improved. AnSi concentration of not more than 1.1×10¹⁹ cm⁻³ has insufficienteffects, and that of not less than 3.0×10¹⁹ cm⁻³ degrades crystalquality.

The spacer layer 143 serves to block the anomalous diffusion due to thedrift of Mg atoms into the light emitting unit 140 caused by that theelectric field by the built-in potential is concentrated in the p-typecladding layer 151 due to the high Si concentration of the n-typesemiconductor layer 130. Thereby, the p-type cladding layer 151 with ahigh-Al composition can have a low resistance without decreasingreliability and efficiency.

An overflow of electrons into the p-type cladding layer 151 can besuppressed because electron concentration proximal to the interface withthe p-type cladding layer 151 decreases. At the same time, although thenumber of nonradiative recombinations at the interface increases due toan increase in hole concentration proximal to the interface, this losscan be held low because the density of dislocation is low and thebarrier layer includes an AlGaInN quarternary mixed crystal (the contentof In being 0.3% to 2%).

The Mg concentration of the p-type cladding layer 151 is higher on thelight emitting unit 140 side and lower on the p-type contact layer 152side. This concentration profile cancels the piezoelectric field in thep-type cladding layer 151 inhibiting the injection of holes, decreasesoperating voltage, and improves the effect of carrier confinement.

If the Mg concentration of the p-type cladding layer 151 proximal to thep-type contact layer 152 is not less than 1×10²⁰ cm⁻³, a diffusion of Mginto the light emitting unit 140 occurs to decrease efficiency andreliability. If not more than 5×10¹⁹ cm⁻³, the operating voltageincreases.

The method for manufacturing the semiconductor light emitting device ofthe above example uses the conductive Si substrate as a support basemember (the second substrate 180) used in the separation process of thefirst substrate 110. However, the invention is not limited thereto. Aninsulator such as ceramics and the like also can be used as the secondsubstrate 180 in a similar way, while utilizing the resistance to crackaccording to the method.

Further, Sn and Ge as well as Si may be used as the n-type dopant. Inparticular, doping Si can provide a high-concentration n-type contactlayer of a thick film, allowing fabricating a device of a low operatingvoltage through reducing series resistance.

In the aforementioned, the buffer layer 120, the GaN layer 123, then-type semiconductor layer 130, the light emitting unit 140, and thep-type semiconductor layer 150 are film-formed in the following manner.

First, NH₃ gas as a group V material and an organometallic Al compoundsuch as Al(CH₃)₃ and Al(C₂H₅)₃ as a group III material are introducedinto a reaction chamber at a substrate temperature of not lower than1050° C. and not higher than 1200° C. to grow a first buffer layer 121made of AlN with uniform crystal orientation on the first substrate 110.

To align the crystal orientation of the first buffer layer 121 made ofAlN, it is important to control the ratio of the group V material andthe group III material to be supplied. To obtain a high-quality filmwithout vacancies, a first ratio of (a supply rate of group V atoms)/(asupply rate of group III atoms) is set to be not less than 0.7 and notmore than 50.

To obtain a sufficient quality with good repeatability, the first ratiois preferably not less than 1.2 and not more than 3.0.

Next, the substrate is heated to a temperature of not lower than 1250°C. and not higher than 1350° C. to grow the second buffer layer 122 madeof AlN, after which the surface is planarized. At this time, a secondratio of (a supply rate of group V atoms)/(a supply rate of group IIIatoms) is set higher than the first ratio for the first buffer layer121. Thereby, a lateral-direction growth is promoted and defects arereduced and flatness is improved.

In the case where AlGaN is used for the second buffer layer 122, anorganometallic Ga compound such as Ga(CH₃)₃ or Ga(C₂H₅)₃ may be added asthe group III material.

More specifically, the first buffer layer 121 is epitaxially grown bylow-pressure metalorganic chemical vapor deposition at a firsttemperature with the first ratio of 0.7 to 50. Then, the second bufferlayer 122 is epitaxially grown by metalorganic chemical vapor depositionat a second temperature higher than the first temperature and with thesecond ratio higher than the first ratio.

Then, the substrate temperature is set not lower than 1100° C. and nothigher than 1250° C. to grow the GaN layer (the lattice relaxationlayer) 123.

After that, nitride semiconductor layers containing Al, In, Ga, and/orthe like which form a device structure unit (the n-type semiconductorlayer 130, the light emitting unit 140, and the p-type semiconductorlayer 150) are stacked thereon.

At this time, an organometallic Ga compound such as Ga(CH₃)₃ andGa(C₂H₅)₃, and an organometallic In compound such as In(CH₃)₃ andIn(C₂H₅)₃ may be used as the group III material, as well as theorganometallic Al compound described above.

In this case, for a dopant, Si hydride such as SiH₄ and/or anorganosilicon compound such as Si(CH₃)₄ may be used as an n-type dopant.An organometallic Mg compound such as Cp₂Mg(bis(cyclopentadienyl)magnesium) or m-Cp₂Mg(bis(methylcyclopentadienyl)magnesium) may be used as a p-type dopant.

Although it has been thought that a heat treatment at a temperature ofabout 800° C. is necessary to remove hydrogen mixed in the growth layerfor increasing the activation rate of the p-type dopant, the generationof N-atom vacancies can be suppressed by the growth with a high ratio of(supply rate of group V atoms)/(supply rate of group III atoms), andtherefore inactivation due to hydrogen can be avoided essentially.Further, the degradation of crystal quality due to the heat treatmentcan be avoided as well.

SECOND EXAMPLE

FIG. 6 is a schematic cross-sectional view in order of the steps,illustrating a method for manufacturing a semiconductor light emittingdevice according to a second example of the present invention.

As shown in FIG. 6, in the method for manufacturing a semiconductorlight emitting device of the second example of the present invention, ann-side electrode 170 is provided on the surface of the n-typesemiconductor layer 130 after the first substrate 110 is separated.

More specifically, after the first substrate 110 is separated off, thesurface of the n-type semiconductor layer 130 is polished to expose then-type contact layer 131.

Then, an SiO₂ film is formed with a thickness of 400 nm by using, forexample, a thermal CVD apparatus.

Subsequently, the n-side electrode 170 is formed in the followingmanner. A patterned resist for resist-lift-off process is formed on then-type contact layer 131 opposing to a portion in which the p-sideelectrode 160 is not formed. The SiO₂ film on the exposed n-type contactlayer 131 is removed by a treatment with ammonium hydrogen fluoride. Astacked film serving as the n-side electrode 170 based on, for example,Ti/Pt/Au is formed with a film thickness of 500 nm in the region wherethe SiO₂ film is removed.

At this time, fine concave and convex 135 may be formed on the surfaceof the n-type contact layer 131, which can enhance light extractionefficiency. Since the surface of the n-type contact layer 131 has polarcharacteristics of a surface of nitrogen, a concave and a convex formedresulting from the crystal defects by the reactive ion etching, forexample, may be used to form the concave and convex 135.

After that, the workpiece is cut by cleavage, a diamond blade or thelike and individual LED devices with a width of 400 μm and a thicknessof 100 μm are formed.

Thus, the semiconductor light emitting device 20 illustrated in FIG. 6can be fabricated.

As described above, the method for manufacturing the semiconductor lightemitting device according to this example can suppress crystal damageduring the separation of the sapphire substrate, and provide asemiconductor light emitting device of short wavelength with highproductivity and high efficiency.

In the specification of this application, the “nitride semiconductor”includes all semiconductors having a chemical formula ofB_(x)In_(y)Al_(z)Ga_(1-x-y-z)N, where 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z≦1,while changing the composition ratios x, y, and z in the respectiveranges. Further, the “nitride semiconductor” includes those furthercontaining a group V element other than N (nitrogen) and/or any ofvarious dopants added to control the conduction types and the like inthe above chemical formula as well.

In the above, the semiconductor light emitting device manufactured bythe method for manufacturing the semiconductor light emitting deviceaccording to the embodiment of the present invention includes an n-typesemiconductor layer, a p-type semiconductor layer, and a light emittingunit provided between the n-type semiconductor layer and the p-typesemiconductor layer. Here, a wafer including an n-type semiconductorlayer, a p-type semiconductor layer, and a light emitting unit providedbetween the n-type semiconductor layer and the p-type semiconductorlayer also can be regarded as a semiconductor light emitting device forconvenience.

Hereinabove, embodiments of the present invention are described withreference to specific examples. However, the present invention is notlimited to these examples. For example, those skilled in the art mayappropriately select specific configurations of components of thesemiconductor light emitting device from known art and similarlypractice the present invention. Such practice falls under the scope ofthe present invention to the extent that similar effects thereto areobtained.

Further, any two or more components of the specific examples may becombined within the extent of technical feasibility. Such combinationfalls under the scope of the present invention to the extent that thespirit of the present invention is included.

Moreover, all methods for manufacturing a semiconductor light emittingdevice that can be obtained by an appropriate design modification bythose skilled in the art based on the method for manufacturing asemiconductor light emitting device of the embodiment of the presentinvention described above also are within the scope of the presentinvention to the extent that the spirit of the present invention isincluded.

Furthermore, various modifications and alternations within the idea ofthe present invention will be readily apparent to those skilled in theart. All such modifications and alternations should be seen as withinthe scope of the present invention.

1. A method for manufacturing a semiconductor light emitting device, thedevice including an n-type semiconductor layer, a p-type semiconductorlayer, and a light emitting unit provided between the n-typesemiconductor layer and the p-type semiconductor layer, the methodcomprising: forming a buffer layer made of a crystalline Al_(x)Ga_(1-x)N(0.8≦x≦1) on a first substrate made of c-plane sapphire and forming aGaN layer on the buffer layer; stacking the n-type semiconductor layer,the light emitting unit, and the p-type semiconductor layer on the GaNlayer; and separating the first substrate by irradiating the GaN layerwith a laser having a wavelength shorter than a bandgap wavelength ofGaN from the first substrate side through the first substrate and thebuffer layer, wherein the p-type semiconductor layer includes a p-typecladding layer provided on the light emitting unit and a p-type contactlayer provided on the p-type cladding layer, and the p-type claddinglayer is Mg-doped p-type AlGaN formed to have a concentration profileincluding a higher Mg concentration on a side of the first substrate anda lower Mg concentration on a side opposite to the first substrate, sothat a piezoelectric field in the p-type cladding layer is canceled bythe concentration profile.
 2. The method according to claim 1, whereinthe forming the buffer layer includes stacking a first buffer layercontaining carbon and a second buffer layer having a lower carbonconcentration than the first buffer layer on the first substrate.
 3. Themethod according to claim 2, wherein the first buffer layer is a layermade of AlN having a carbon concentration of not less than 3×10¹⁸ cm⁻³and not more than 5×10²⁰ cm⁻³, and a thickness of not less than 3nanometers and not more than 20 nanometers.
 4. The method according toclaim 2, wherein the second buffer layer is a layer made of AlN having acarbon concentration of not less than 1×10¹⁶ cm⁻³ and not more than3×10¹⁸ cm⁻³, and a thickness of not less than 0.6 micrometers and notmore than 6 micrometers.
 5. The method according to claim 2, wherein thefirst buffer layer is epitaxially grown by low-pressure metalorganicchemical vapor deposition at a first temperature with a first ratio of(a supply rate of group V atoms)/(a supply rate of group III atoms) of0.7 to 50, and the second buffer layer is epitaxially grown bymetalorganic chemical vapor deposition at a second temperature higherthan the first temperature with a second ratio of (a supply rate ofgroup V atoms)/(a supply rate of group III atoms) higher than the firstratio.
 6. The method according to claim 1, wherein the buffer layer ismade of AlN.
 7. The method according to claim 1, wherein the bufferlayer has a film thickness of 1 micrometer to 4 micrometers.
 8. Themethod according to claim 1, wherein the buffer layer has a higherthermal conductivity than the GaN layer.
 9. The method according toclaim 1, wherein the n-type semiconductor layer includes an Si-dopedn-type GaN layer provided on the GaN layer, and an Si-doped n-type AlGaNlayer provided on the Si-doped n-type GaN layer.
 10. The methodaccording to claim 9, wherein the Si-doped n-type GaN layer has asilicon concentration of not less than 1×10¹⁹ cm⁻³ and not more than2×10¹⁹ cm⁻³, and the Si-doped n-type AlGaN layer has a siliconconcentration lower than the silicon concentration of the Si-dopedn-type GaN layer.
 11. The method according to claim 1, wherein the lightemitting unit includes a multiple-quantum-well structure having barrierlayers and well layers alternately stacked.
 12. The method according toclaim 11, wherein the barrier layer is a Si-doped n-type AlGaInN and hasa silicon concentration of 1.1×10¹⁹ cm⁻³ to 3.0×10¹⁹ cm⁻³.
 13. Themethod according to claim 1, wherein the p-type contact layer isMg-doped p-type GaN and has a lower Mg concentration on a side of firstsubstrate than on a side opposite to the first substrate.
 14. The methodaccording to claim 1, wherein the device further includes a spacer layerprovided between the light emitting unit and the p-type semiconductorlayer, and the spacer layer includes AlGaInN and has an Si concentrationof 1×10¹⁶ cm⁻³ to 3.0×10¹⁸ cm⁻³.
 15. The method according to claim 1,wherein a peak wavelength of emitted light of the light emitting unit islonger than 370 nanometers and shorter than 400 nanometers.
 16. Themethod according to claim 1, wherein a second substrate is bonded on aside of the p-type semiconductor layer opposite to the light emittingunit before irradiating the laser.
 17. The method according to claim 16,wherein an electrode is formed on the side of the p-type semiconductorlayer opposite to the light emitting unit before bonding the secondsubstrate.
 18. The method according to claim 1, wherein projections ordepressions are formed on a surface on a side of the p-typesemiconductor layer opposite to the first substrate at a spacing notlonger than a wavelength of emitted light of the light emitting unit.19. The method according to claim 1, wherein the forming the bufferlayer includes forming a first buffer layer made of crystalline AlN onthe first substrate and forming a second buffer layer made ofcrystalline Al_(x)Ga_(1-x)N (0.8≦x<1) having a lower carbonconcentration than the first buffer layer on the first substrate. 20.The method according to claim 19, wherein the light emitting unitincludes a multiple-quantum-well structure having barrier layers andwell layers alternately stacked, and the barrier layer includes anAlGaInN quaternary mixed crystal having an In content not less than 0.3%and not more than 2%.